Gas turbines coupled to electric generators are commonly used in power generation service. It is desirable to optimize operation of a gas turbine in order to provide reliable power generation while reducing undesirable emissions from the combustion process in the turbine. For example, the gas turbine combustion process results in generation of, among other things, nitrogen oxides (NO.sub.x), unburnt hydrocarbons, and carbon monoxide (CO). Control of such undesirable emissions requires control of the fuel-air ratio (FAR) of the combustible mixture being fed into the combustion chamber of the turbine. One approach to minimize emissions has been to design a turbine so that when it is operating at full load conditions the FAR has an equivalence ratio (actual FAR divided by the stoichiometric FAR) that corresponds to a desired point between the lean burnout point (when turbine combustor flameout occurs because the FAR is too lean) and the rich burnout point (flameout due to FAR being too rich). For emission and fuel economy reasons, turbines are commonly operated with a fuel air mixture for which the equivalence ratio is less than one (that is, leaner than the stoichiometric FAR).
Controllers for gas turbines typically employ a decentralized control strategy in which fuel supply to the turbine and air supply to the turbine are controlled by reference to different measured turbine performance parameters (typically represented by respective turbine condition signals). For example, in a typical gas turbine controller, fuel supply to the turbine is controlled primarily via a feedback loop that seeks to match turbine power output with the electrical load demand on the generator driven by the turbine. This feedback is typically accomplished through monitoring turbine speed, with a speed error signal (that is variation of the measured turbine speed with a reference (or set point) value) being processed to increase or decrease fuel supply to the turbine as appropriate.
Air supply to the turbine in such a system is determined by the compressor inlet geometries which are controlled based on the error between measured turbine exhaust temperature and a reference temperature value; the compressor inlet guide vanes are positioned to increase or decrease air flow into the turbine as necessary to obtain the optimal exhaust temperature. In such a control system, a change in load on the turbine results in a substantially immediate change in fuel flow to the turbine, leading to a change in the power output and exhaust temperature, which change in exhaust temperature causes the controller commanding the inlet guide vanes to appropriately change air flow to the turbine. There is thus a lag between fuel supply control and air supply control. Systems can be designed such that the lag can be acceptable for some range of turbine operating conditions.
The control system lag presents significant challenges, however, in the event of a loss of load condition occurs (e.g., an electrical breaker opens) unloading the electrical generator driven by the turbine. Due to the magnitude of the change in load, the control system lag (that is, the delay in adjusting air supply to the turbine due to the exhaust temperature feedback loop) may cause one or more combustors in the turbine to flame out due to the fuel-air mixture becoming too lean to sustain combustion (too much air admitted for amount of fuel being supplied). Flameout of a sufficient number of combustors results in the turbine control system shutting the turbine down for safety reasons. Such a shutdown takes the turbine off line and necessitates the initiation of a restart process that delays reloading the turbine. The flameout also imposes thermal and mechanical cycles on the turbine that are desirably avoided.
It is thus desirable that a turbine control system be able to control a gas turbine so that the turbine is maintained in an operating condition (e.g., it does not flame out) even in the event of a full load rejection condition.